An accelerometer is one of the primary sensors used in on-board automotive safety control systems and navigational systems, particularly crash sensing systems. Examples of such automotive applications include anti-lock braking systems, active suspension systems, supplemental inflatable restraint systems such as air bags, and seat belt lock-up systems. An accelerometer is a device which measures acceleration, or more accurately, the force that is exerted by a body as the result of a change in the velocity of the body. A moving body possesses inertia which tends to resist the change in velocity. It is this resistance to any change in velocity that is the source of the force which is exerted by the moving body. This force is directly proportional to the acceleration component in the direction of movement when the moving body is accelerated.
In the past, electromechanical and electronic accelerometers have been widely used in the automotive industry to detect an automobile's deceleration. More recently, micro-machined accelerometers which employ piezoresistive microbridges have been used. Such accelerometers detect acceleration in the plane perpendicular to a plane through a proof mass and the microbridge which supports the proof mass. Acceleration and deceleration of the vehicle cause a compressive or tensile load on piezoresistive elements embedded in the microbridge which supports the proof mass, depending on which direction the acceleration or deceleration is applied within that plane. It is the accelerating force on the support system for the proof mass, and the proof mass inertia, which generates compressive or tensile loads on the piezoresistive elements. In turn, the resulting compressive and tensile loads change the electrical resistance of the piezoresistive elements embedded in the microbridge. This change in electrical resistance can be sensed to determine the magnitude of the acceleration component perpendicular to the plane of the common axis shared by the piezoresistive elements.
Piezoresistive accelerometers are capable of extremely precise measurements, and are therefore desirable for use in automotive applications. However, they must be adequately packaged to protect the micro-machined accelerometer from an automobile's harsh environment. In particular, the accelerometer must be isolated from the mechanical stresses with its mounting package, and reasonably isolated from the extraneous road and vehicle vibrations during use. For example, the packaging for the accelerometer must be sufficiently rigid such that its resonant frequency lies above the frequency range to be detected. An accelerometer intended to detect acceleration or deceleration of a vehicle would require that it be insensitive to jarring of the vehicle due to rough road surfaces, low speed collisions, and the like.
The accelerometer's packaging must also isolate the accelerometer from the harsh automotive environment, such as salt, grease, dust and moisture, yet be easy to assemble in order to reduce material and assembly costs, such that the accelerometer package is amenable to high volume, low cost automotive production techniques. The above features are complicated by the desire for accelerometer packaging to be small and compact, while also providing an inert protective atmosphere for the micro-machined accelerometer. One approach has been to form the accelerometer package by overmolding the accelerometer with a suitable plastic material. However, a drawback of this approach is the significant package-induced stresses which are created as a result of the different coefficients of thermal expansion for the materials used.
An accelerometer package which overcomes the above shortcoming is taught by U.S. Pat. No. 5,233,871 to Schwarz et al., assigned to the assignee of this invention. Schwarz et al. teach a microaccelerometer package in which a micro-machined accelerometer and its associated signal conditioning and processing circuitry are packaged within a three piece housing in a manner which minimizes space requirements, yet does not involve overmolding the accelerometer. The accelerometer and the signal processing circuitry are positioned and secured relative to each other on a pair of recessed surfaces within the package, such that each is rigidly secured to the package and is isolated from extraneous vibrations which are transmitted to the package from the environment. During processing, the signal processing circuitry advantageously remains accessible for final trimming and tuning procedures prior to sealing the package. The accelerometer and the signal processing circuitry are also positioned relative to each other so as to minimize the length of the electrical conductors required to electrically connect them, which facilitates automated procedures for attaching the electrical conductors to their wire bond sites.
Though the packaging taught by Schwarz et al. provides a small compact assembly which is amenable to automotive production techniques, it would be highly desirable if further reductions in packaging size and manufacturing complexity and costs could be achieved. Therefore, it would be advantageous to provide a microaccelerometer package whose construction is able to substantially isolate its micro-machined accelerometer from extraneous vibrations and package-induced stresses. In addition, it would be desirable if the package were rigid and required a minimal number of components so as to facilitate its assembly, and therefore minimize assembly costs. Furthermore, it would be desirable if the size of the support structure for mounting the package within its environment could be reduced, so as to minimize the overall size of the package.